Loudspeaker drivers convert an electrical signal to sound by actuating an electromagnet connected to a piston which vibrates in air; sound emanates from both sides of the driver's piston, but while the waves are equal in force they are opposite in phase. So, the maximum audible power obtainable from a loudspeaker is one-half of the total sound energy at the piston, and in practice it is usually much less than that. Further, where these waves meet, they have the opportunity to cancel one another by means of destructive wave interference. A purpose, therefore, of isolating and neutralizing the sound from one side of the driver, is to prevent the energy from that rearward wave from impinging in any way upon the sound projecting forward by the initial action of the driver. Negative effects can manifest by a variety of means, including interference, echoing within the enclosure returning to bounce against the piston, re-radiation through the enclosure walls, setting the enclosure walls into vibration as if they were secondary pistons, as well as port effects. These can reduce the overall performance of the loudspeaker, e.g., by reducing the efficiency of sound production, distorting the sound as compared to the input signal, and phase shifting certain frequencies to cause the tones to be less distinct.
There are two major design groups for audio loudspeakers: sealed and vented. The sealed group specifies a fixed quantity of air that is held in the enclosure for any given driver, and is subdivided into “infinite baffle” and “acoustic suspension” types. Vented loudspeakers employ substantial containment with a port or other means by which a certain quantity of the contained air volume is exchanged with air outside of the enclosure in response to the piston action, and is subdivided into “bass-reflex” and “passive radiator” types.
Infinite baffle is used extensively by driver manufacturers in testing and producing performance curves for their products. One way infinite baffle is manifested is to construct a heavy, soundproofed wall between two rooms and mount a driver in a hole on that wall. The baffle room, behind the driver, is large enough such that no sound wave projecting rearward meets with any surface such that it can reflect and return to the piston surface with enough force to have any measurable effect on the sound projecting into the test room. The test room, forward of the driver, is usually constructed so as to be anechoic, such that sounds measured in the test room are those emanating only from the driver, without room absorption or reflection affecting the sound production. The infinite baffle enclosure may create an environment in which a driver can perform at or near its full capability, and performance curves and specifications obtained in this environment may provide a uniform basis from which loudspeaker designers and engineers can compare drivers from different manufacturers with a reasonable expectation that the results are direct comparisons. The arrangement described is usually a permanent installation, as room sizes are on the order of 1,000 cubic feet or more. Infinite baffle is also possible in a portable enclosure which may simply be a sealed box where the enclosed volume is large enough such that its pneumatic compliance Cab (defined as being inversely proportional to the spring constant of the enclosed air volume) is greater than the suspension compliance specification Cas of the driver mounted in it. Cas is also an inverse relation, in this case of the spring constant established by the surround and spider suspending the piston in the driver's frame. The relation of Cas to Cab provides for a finite enclosure wherein the air volume resonance has minimal effect on the action of the piston. The portable infinite baffle enclosure was used extensively in early electronic audio but due to the large box sizes needed, as well as inherent performance limitations, it has largely been superseded by other types of enclosures.
The acoustic suspension enclosure is a sealed box design which may define the compliance ratio of Cas to Cab so that the contained air volume becomes an integral part of the driver's suspension. This design may be characterized as having good fidelity to the original recording, smaller enclosures than infinite baffle, and being relatively easy to build. Some drawbacks of this enclosure type are problems associated with containment such as echoes, standing waves, and the slow decay of low frequency tones. As frequency declines, wavelength and the total quantity of energy carried by a single note both may increase. The acoustic suspension's sealed environment may depend largely on time to degrade wave energy, which permits low frequency energy to disrupt sound production while declining in intensity. To obtain acceptable bass production, acoustic suspension loudspeakers may require relatively large diameter drivers which may represent a larger cost. For example, a 12-inch diameter driver may be necessary to obtain a lower limit of 60 Hz using acoustic suspension technology where an 8-inch driver may be able to produce the same 60 Hz in a bass-reflex enclosure.
A. C Thuras was granted a patent in 1932 for a loudspeaker having a vent to exchange air in response to the piston action of the driver. Much effort from many researchers over the next several decades, including Thiele (1961) and Small (1973) for whom the industry standard “Thiele/Small Parameters” are named, resulted in a systemization of the variables associated with both of the major groups of loudspeaker design. This was especially valuable to designers of bass-reflex systems because of the increased complexity. In the absence of a driver, the bass-reflex enclosure may act as a Helmholtz resonator wherein the relative masses of the enclosed air and the air occupying the port tube move together to establish a frequency at which the device resonates in response to an impulse. Any such resonator may have one characteristic frequency. An advantage of introducing box resonance to the loudspeaker may include a quicker way to dissipate the energies associated with long, powerful sound waves contained by the enclosure: e.g., the Helmholtz resonant action dissipates energy through the port. A carefully tuned bass-reflex loudspeaker may produce sound frequencies as low as 20 Hz (the lower limit of human hearing) in a box that may be relatively easy to manufacture, move and install, e.g., with fairly good sound fidelity. Using the same driver, bass-reflex designs may produce frequencies as much as a full octave deeper in tone than a sealed unit. They can be more complex to engineer but the cost of production is a minimal increase once a design is established, and this may account for the tremendous popularity of the design type. A problem with vented design is the inclusion of box resonance as a dynamic element of sound reproduction: the energy dissipated through the port is typically not a genuine representation of the electrical signal at the driver, yet it is heard along with the sound from the forward side of the driver. All other variables being equal, acoustic suspension is generally recognized as superior in fidelity and overall sound quality. Bass-reflex may be a far more efficient means of obtaining deep bass tones, and this dichotomy may represent the current state of the art.
It is commonly understood that a box having equal side lengths may support standing waves at some frequencies but not others, which may cause undesirable fluctuations in the frequency response curve. To reduce standing wave effects, traditional designs depend on some degree of asymmetry and difference of dimension. For example, in a common case of a rectangular box, the length, width and height are usually different values, and sometimes specified in certain ratios to each other. Ported enclosures may permit air to pass back and forth from outside ambient space to inside the box, and in doing so incrementally change the mass of air acting against the back of the piston. These small changes may cause phase delays in the damping response that vary with frequency. There are known methods designed to enhance the performance of the bass-reflex concept, such as those found in Loudspeaker Design Cookbook by Vance Dickason. A successful bass-reflex design may depend on the relation of the box resonance F3 to the driver's fundamental resonant frequency Fs. These two values may be balanced by tuning the diameter and length of the port tube. A partial fill of fiberglass insulation as a damping material may also be used. The placement of a passive radiator, essentially an unpowered driver, in place of the port, is another technique for minimizing port effects, using the passive radiator's suspension to modulate the exchange with ambient conditions. These methods may reduce the negative effects of exchanging mass with ambient air but may not completely account for the loss of fidelity.
In this context Fs, a Thiele/Small parameter, may refer to a property of the driver. It may represent the free-air resonant frequency of the driver piston in Hertz, measured without an enclosure, and may represent the lowest frequency at which a given driver operates at full efficiency. F3 may refer to a common term of art and refer to, e.g., the “minus 3 decibel roll-off” frequency in a completed loudspeaker's SPL performance curve. Because the endpoints of a loudspeaker's performance may not be distinct vertex corners, e.g., at the bass end, after establishing a straight-line average number (e.g. 96 dB), the frequency at which the performance “rolls off” to 3 dB less than the average (in this case 93 dB) may be specified as F3. For the purposes of loudspeaker production F3 may be used as an agreed upon point establishing the lower limit of a given completed unit's capability.
Contemporary loudspeaker engineering may relate box volume to the parameter referred to as Vas, or compliance equivalent volume. Vas indicates the volume of air in a sealed container having the same “springiness” as the driver under test has in free air. Driver manufacturers typically publish specifications listing the “Thiele/Small Parameters” for each driver available for sale to assist loudspeaker design-engineers in selecting units to be used in a given loudspeaker. Box volume Vb determination in acoustic suspension design may start with the Thiele/Small parameter Vas and divide it by a factor α obtained from a table calculated to maximize certain characteristics, for example, highest fidelity, maximum bass, maximum damping, or maximum power handling. Values for α may vary from one half to thirty; a common value for acoustic suspension may be three, while for bass-reflex it may be 1.3. The tables were developed by Small and resulted in a quick way for an engineer to obtain a working value for Vb based on compliance ratios, and there are separate tables for sealed and vented loudspeakers. More recently, computer algorithms have been developed for use by designers of loudspeaker systems, into which many Thiele/Small parameters can be fed and which calculate an optimum starting point for Vb. While corrections may be made for maximum piston excursion, driver resonance, effective piston diameter, and the Q factors for electrical and mechanical efficiency, the factor that is often given the greatest weight in the calculation is still Vas, the measure of suspension compliance.
FIG. 1 illustrates a traditional speaker design, in which a driver 100 is placed somewhat off-center within an enclosure 101 to counter the potential effects of the types of standing waves within the enclosure. Any off center mounting will align the piston 102 with a greater mass of air V2 122 on one side of the excursion axis 103 than the other side V1 121, and therefore may result in unequal return elasticity from the differing air masses. This can result in incremental differences in the pneumatic pressures across the piston 102, which can cause distortions in the surface resulting in decreased sound quality. In some cases the pneumatic imbalance can cause lateral movement of the electromagnetic coil in the magnet gap resulting in knocking and/or distortion.
Excursion is defined as the extent of linear motion traveled by the piston along the excursion axis 103 of the electromagnetic coil. The rapid back and forth motion on this path is the vibration coupling the electrical signal to the local air masses V1 121 and V2 122, and creates the waves which propagate as sound. Maintaining a balance of forces surrounding the excursion axis 103 can have a substantial effect on the ability of the piston 102 surface to create sinusoidal wave patterns. Because of this, much of the engineering effort that goes in to manufacturing high quality drivers concerns balancing the force of the suspension with air gap tolerances in the coil to provide protection against lateral movement while permitting adequate excursion for a given applied force. Asymmetry in the contained air volume can disturb the balance designed into a driver and cause distortions due to the force imbalance.
Traditional design methods seek to balance the performance gain of asymmetry with respect to standing waves against the loss of asymmetry due to pneumatic losses by depending on suspension rigidity within the driver's “spider” to keep the voice coil centered. The improvement in the sound pressure level (“SPL”) response curve, however, is countered by increases in distortion, reduction of the dynamic range of the system, and reduced total power handling capability.